N-(phosphonomethyl)glycine (glyphosate) is described by Franz in U.S. Pat. No. 3,799,758. N-(phosphonomethyl)glycine and its salts are conveniently applied as a component of aqueous, post-emergent herbicide formulations. As such, they are particularly useful as a highly effective and commercially important broad-spectrum herbicide for killing or controlling the growth of a wide variety of plants, including germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation and aquatic plants.
One of the more widely accepted methods of making N-(phosphonomethyl)glycine products includes the catalyzed liquid phase oxidative cleavage of a carboxymethyl substituent from an N-(phosphonomethyl)iminodiacetic acid (PMIDA) substrate. Over the years, a wide variety of methods and reactor systems have been disclosed for conducting this oxidation reaction. See generally, Franz, et al., Glyphosate: A Unique Global Herbicide (ACS Monograph 189, 1997) at pp. 233-62 (and references cited therein); Franz, U.S. Pat. No. 3,950,402; Hershman, U.S. Pat. No. 3,969,398; Felthouse, U.S. Pat. No. 4,582,650; Chou, U.S. Pat. No. 4,624,937; Chou, U.S. Pat. No. 4,696,772; Ramon et al., U.S. Pat. No. 5,179,228; Siebenhaar et al., International Publication No. WO 00/01707; Ebner et al., U.S. Pat. No. 6,417,133; Leiber et al., U.S. Pat. No. 6,586,621; and Haupfear et al., U.S. Pat. No. 7,015,351.
For example, such reaction may be conducted in either a batch or continuous oxidation reactor system in the presence of a catalyst that typically comprises particulate carbon, or a noble metal such as platinum on a particulate carbon support. The catalyst is usually slurried in an aqueous solution of PMIDA within a stirred tank reactor, and molecular oxygen introduced into the reactor to serve as the oxidizing agent. The reaction is exothermic. The liquid phase oxidation of a PMIDA substrate typically produces a reaction mixture containing water and various impurities besides the desired N-(phosphonomethyl)glycine product. These impurities may include, for example, various by-products, unreacted starting materials, as well as impurities present in the starting materials. Representative examples of impurities present in N-(phosphonomethyl)glycine product reaction mixtures include unreacted PMIDA substrate, N-formyl-N-(phosphonomethyl)glycine, phosphoric acid, phosphorous acid, hexamethylenetetraamine, aminomethylphosphonic acid (AMPA), methyl aminomethylphosphonic acid (MAMPA), iminodiacetic acid (IDA), formaldehyde, formic acid, chlorides and the like.
Commercial considerations sometimes dictate that the concentration of the N-(phosphonomethyl)glycine product in the commercially sold mixtures be significantly greater than the concentrations in the reaction mixtures that are typically formed in the oxidation reactor system, particularly where the N-(phosphonomethyl)glycine product is being stored or shipped for agricultural applications. For example, when a heterogeneous catalyst is used for the liquid phase oxidation of PMIDA to make N-(phosphonomethyl)glycine as described by Haupfear et al. in U.S. Pat. No. 7,015,351, it is typically preferred to maintain a maximum concentration of the N-(phosphonomethyl)glycine product in the reaction mixture of no greater than about 9% by weight in order to keep the product solubilized, although higher concentrations in excess of 9% and even up to about 12% by weight may be suitably utilized at higher reaction mixture temperatures. Sometimes, however, it is desirable for the commercially sold mixtures to have an N-(phosphonomethyl)glycine concentration that is significantly greater. Thus, after the N-(phosphonomethyl)glycine product has been formed and, if necessary, separated from the catalyst, it is often preferred to concentrate the product and separate the product from the various impurities in the oxidation reaction mixture.
Concentration of the glyphosate product typically comprises one or more crystallization steps. The value of the N-(phosphonomethyl)glycine product normally dictates maximal recovery of the product from the reaction mixture and also often provides incentive for recycling at least a portion of the depleted reaction mixture (i.e., crystallization mother liquor). The mother liquor stream or streams obtained in the crystallization may be recycled to crystallization or reaction steps of the process. A fraction of the mother liquor(s) is generally removed from the process in order to purge by-products and control the purity of the N-(phosphonomethyl)glycine product. The crystallized N-(phosphonomethyl)glycine product may be dried and sold as a solid crystalline product. A substantial fraction of the glyphosate crystals are commonly neutralized with a base such as isopropylamine, KOH, etc. in an aqueous medium to produce a concentrated salt solution. A concentrated formulation comprising the glyphosate salt solution, and often also other components such as, for example, various surfactants, is a principal product of commerce.
Haupfear et al., in U.S. Pat. No. 7,015,351, describe various processes for purifying and concentrating an N-(phosphonomethyl)glycine product solution prepared by the oxidation of a PMIDA substrate. Haupfear et al. disclose generating two crystalline N-(phosphonomethyl)glycine products (i.e., wet-cakes) in two separate crystallizer trains operated in semi-parallel, one train including an adiabatic crystallizer and the other including a heat-driven evaporative crystallizer. The wet-cake products have distinct impurity profiles and the lower purity material issuing from the heat-driven evaporative crystallizer train may be combined with the higher purity material issuing from the adiabatic crystallizer train to produce a single product of acceptable purity.
Donadello, in Italian Patent No. 1281094, describes a process for removal of formaldehyde from N-(phosphonomethyl)glycine reaction mixtures comprising N-(phosphonomethyl)glycine or salts thereof and produced by the catalytic oxidation of N-(phosphonomethyl)iminodiacetic acid or salts thereof. In one embodiment, mother liquor from the crystallization of the N-(phosphonomethyl)glycine reaction mixture is subjected to selective membrane separation utilizing a reverse osmosis or nanofiltration membrane to produce a permeate containing formaldehyde and a concentrate or retentate enriched in N-(phosphonomethyl)glycine. The retentate can be subjected to crystallization to recover residual N-(phosphonomethyl)glycine and the resulting mother liquor recycled to the selective membrane separation step.
Vandenmersch et al., in U.S. Pat. No. 7,071,354, describe a process for recovery of N-(phosphonomethyl)glycine from aqueous mixtures containing N-(phosphonomethyl)glycine, ammonium halides and alkali metal or alkaline earth metal halides. Rather than oxidation of an N-(phosphonomethyl)iminodiacetic acid substrate, the aqueous mixture preferably originates from the reaction of a hexahydrotriazine derivative and a triacyl phosphite and is obtained after precipitation and recovery of the N-(phosphonomethyl)glycine product. The process includes adjusting the pH of the aqueous mixture from 2 to 8 and subjecting the mixture to a separation on a selective nanofiltration membrane to produce a retentate and a permeate said to be enriched in N-(phosphonomethyl)glycine and halides, respectively. Depending on its concentration and purity, the retentate can optionally be concentrated (e.g., by distillation or reverse osmosis) to obtain N-(phosphonomethyl)glycine in crystalline form that can be recovered in a customary manner, such as by filtration.
Vigil et al., in U.S. Publication No. U.S. 2005/0035060 A1 disclose a process for removing formaldehyde and formic acid impurities from N-(phosphonomethyl)glycine reaction solutions originating from the oxidation of N-(phosphonomethyl)iminodiacetic acid. The process includes providing a N-(phosphonomethyl)glycine solution containing between 0.1% and 3% w/v N-(phosphonomethyl)glycine, 0.5% to 1% w/v formaldehyde, and 0.1% to 0.6% formic acid; adjusting the pH of the initial N-(phosphonomethyl)glycine solution to between 2.5 and 3.5 with a base such as alklyamine, ammonium hydroxide, sodium or potassium hydroxide; contacting the solution with a nanofiltration membrane at a temperature between 10° C. and 35° C. and a pressure between 25 and 35 kg/cm2; recycling the retentate solution to the nanofiltration membrane; and discarding the permeate solution containing the impurities. After successive recycling, a glyphosate concentration of up to approximately 8% is obtained in the recovered retentate solution.
There remains a need for improved processes for concentrating and purifying N-(phosphonomethyl)glycine product in aqueous process streams that utilize selective membrane separation techniques. There is particular need for such processes capable of reducing the operating costs associated with concentrating and precipitating the N-(phosphonomethyl)glycine product and which effectively use selective membrane separation to maximize recovery of the product from aqueous process slurries comprising N-(phosphonomethyl)glycine product crystals and a mother liquor.